The spectrum of solar irradiance can be described as black body radiation with a characteristic temperature of 5800 K. Taking into account the transmission window of visibility of the eye, which cuts off near 350 nm and extends to the infrared and the absorption of atmospheric gases, light available naturally for vision extends from 400 nm to about 1300 nm. Of the available light, the eye generally uses a relatively narrow spectral range for vision. For example, scotopic vision provided for rod cells is centered at about 505 nm and ranges from 435-600 nm. The rods allow for vision under low-light conditions. On the other hand, cones are responsible for color vision and provide vision under conditions of high luminosity. Photopic vision provided by the cones is centered near 550 nm and extends from about 450 to 650 nm. The wavelength region above that in the visible range detected by the rods and cones is known as the infrared.
There is a demand for methods that allow humans and other animals to see in the infrared region of the spectrum, under low light conditions, and/or at wavelengths that cannot be observed by unaided eyes. At present, for example, the military equips soldiers with devices to allow them night vision capabilities. The devices are expensive, often costing $1,000 or more per soldier. Assault aircrafts, helicopters, tanks, and other vehicles are equipped with yet more expensive versions of infrared viewing equipment.
A conventional infrared viewer and night vision scope contains imaging optics, a phosphor screen containing materials that through absorption of infrared photons convert that energy into the visible, an image intensifier that amplifies the un-converted light, and additional optics to render a clear picture for the user. The units are generally heavy, cumbersome, expensive, and fragile, have a very narrow field of view, have no active focusing or light intensity regulation, and require batteries or other power supplies.
The process of up-conversion refers to the conversion of one or more photons of longer wavelengths (less energy) to one or more photons of a shorter wavelength (more energy). Typically, the process proceeds by a two-photon absorption followed by one-photon emission. For example, 2 photons of 800 nm light may be absorbed followed by emission of a visible photon with a wavelength of 400 nm. The probability of two-photon absorption depends on a number of parameters; most importantly of which is a resonance at either the one or two photon levels. When there is no resonance at the one-photon level, it is necessary for two photons to coincide at the up-conversion particle. This typically requires high intensity light, for example light from pulsed laser sources or illumination from non-classical light resulting from two photon down conversion.
Up-conversion can be made much more efficient when there is a one photon resonance that is long lived. In these cases, the light intensity required for two-photon absorption is greatly reduced. In the presence of a one photon resonance, the timing between photons arriving at the up-conversion particles should equal the lifetime of the intermediate state, which can be as long as a microsecond.
Recently, a number of up-conversion materials, also called phosphors, have been produced. A typical up-conversion material involves a sensitizer (a compound that has an intermediate electronic state in near resonance with the wavelength that needs to be up-converted) and an emitter (a compound that accepts the energy from the sensitizer and emits visible light). Sensitizers may involve single or combinations of lanthanoid ions such as Yb3+, which has a resonance near 1000 nm. Additionally, semiconductor materials such as Si, GaAs, GaN, Ge, InN, and ZnS, having band gaps in the infrared may be used.
Targeting of nanoparticles and other sensor molecules such as dyes or therapeutic compounds to specific cells is an active area of research. It has been demonstrated how to target specific cells, for example, for cancer treatments. Such methods include antibody targeting, aptamers, and recombinant viruses.
For delivery to the eye, eye drops are generally ineffective because there is a barrier that prevents penetration of foreign substances from the outer layers of the eye to the interior. Other methods, such as ocular injection or implantation, are less desirable because of fear, discomfort, inflammation, and other side effects in the patient.
It would be desirable to provide methods for enhancing night vision that overcome the limitations of existing night vision equipment. It would also be desirable to provide up-conversion materials and methods for their delivery directly to the eye.